A SEARCH FOR PHOTOMETRIC VARIABILITY IN L- AND T-TYPE BROWN DWARF ATMOSPHERES

Abstract

Using the Gemini infrared camera on the 3-meter Shane telescope at Lick\nObservatory, we have searched for broad-band J and K' photometric variability\nfor a sample of 15 L and T-type brown dwarfs, including 7 suspected spectral\nbinaries. Four of the dwarfs - 2MASS J0939-2448, 2MASS J1416+1348A, 2MASS\nJ1711+2232, and 2MASS J2139+0220 - exhibit statistically significant variations\nover timescales ranging from ~0.5 hr to 6 days. Our detection of variability in\n2MASS J2139+0220 confirms that reported by Radigan et al., and periodogram and\nphase dispersion minimization analysis also confirms a variability period of\napproximately 7.6 +/- 0.2 hours. Remarkably, two of the four variables are\nknown or candidate binary systems, including 2MASS J2139+0220, for which we\nfind only marginal evidence of radial velocity variation over the course of a\nyear. This result suggests that some spectral binary candidates may appear as\nsuch due to the blending of cloudy and non-cloudy regions in a single "patchy"\natmosphere. Our results are consistent with an overall variability fraction of\n35 +/- 5 %, with no clear evidence of greater variability among brown dwarfs at\nthe L dwarf/T dwarf transition.\n